It is known to make cylinder blocks for internal combustion engines from a single casting which has cylinder walls defining a line of cylinder bores and an outer structure which surrounds the cylinder walls and has passages in it for breathing and oil drainage. A space between the cylinder walls and the outer wall structure provides a water jacket having water circulating through it too cool the engine. The cylinder head, usually another casting, is attached to the block by means of a number of fasteners which engage With the block, usually by means of is threads, in the outer wall structure.
Because of the very high pressures produced in the cylinders during combustion, particularly in diesel engines but also in petrol or other internal combustion engines, the force required to hold the head and block together can be very high, reaching about 5 tons per bolt. Analysis of the distribution of pressure in cylinder head gaskets shows that the pressure tends to be greatest around the top of the cylinder walls.
It has been recognized that the cylinder head fasteners tend to distort the cylinder walls, and various block designs have been used to try to overcome this problem. For example GB 2 099 075 describes the provision of an annular flange around each cylinder into which the cylinder head fasteners are threaded. The flange is low down the cylinder so that the region of the cylinder above it is relatively free of distortion. However the clamping forces in the bolts will still be transmitted radially into the bottom of the cylinders causing some distortion even above the flange. U.S. Pat. No. 4,587,933 attempts to solve the problem by distributing the cylinder head bolts evenly about each cylinder and by supporting the base of the cylinders with transverse struts.
The present invention aims to reduce bore distortion by taking an approach which is different to those described in the prior art.
In order to help understanding of the invention, one of the reasons for bore distortion will first be explained more fully with reference to FIG. 1, in which the effect of the high forces in some known blocks is shown schematically. The load path between the fasteners 10 and the cylinder walls 12 is redial and the upward force on the fasteners 10 and the downward force on the cylinder walls 12 result in a moment in the block structure as shown by the arrows 14 which tends to push the cylinder walls 12 inwards near the top of the fastener threads 16, and pull them outwards near the bottom of the fastener threads. This causes distortion of the cylinder bores and results in poor sealing between the piston rings and the bores, which in turn causes an increase in undesirable emissions from the engine.